A modern wireless communication device includes a front-end stage, typically having at least a power amplifier and a matching network, coupled to an antenna for sending and/or receiving radio frequency (RF) signals. A directional coupler may be located between the front-end stage and the antenna, where it enables reading of forward and reverse power levels and/or detection of load mismatch conditions of the antenna.
A conventional directional coupler used in a mobile wireless communication device, such as a cellular telephone, is typically compact in size and shows sufficient directivity over 20 dB through well constructed mutual inductance and capacitance. However, because the directional coupler has one coupled line for coupling to a main line, and its coupling factor is proportional to frequency of the RF signal, the directional coupler experiences excessive power loss in higher frequency ranges due to strong coupling. The length of the directional coupler may be made shorter to avoid the strong coupling and excessive power loss in the higher frequency range, but this generally results in inadequate coupling in lower frequency ranges. Therefore, the directional coupler is used within a limited bandwidth, determined in part by the length of the directional coupler, even when the corresponding coupling factor satisfies the desired criteria. The limited bandwidth makes it difficult to cover entire frequency bands of various communications standards, such as long-term evolution (LTE), which covers a total frequency band from 0.69 GHz to 4.00 GHz, for example.